1. Field of the Invention
The present invention relates to an infrared detector including quantum dot layers, and particularly relates to an infrared detector which realizes a reduction in dark current.
2. Description of the Related Art
In recent years, a quantum dot infrared photo detector (QDIP), which uses a quantum dot structure as an infrared absorbing section has attracted much attention as an infrared detector.
In the quantum dot infrared photo detector, by exciting an electron located at a conduction-band quantum level in an InAs quantum dot sandwiched between i-type GaAs layers by infrared light incident from the outside and catching it as a current, that is, a photocurrent, the infrared light is detected.
Quantum dots composing the QDIP are formed by a self-formation method in a molecular beam epitaxial apparatus, and therefore they are distributed in a plane perpendicular to a growth direction. Note that to increase the efficiency of detecting infrared light, in usual use, layers each including quantum dots are stacked in plural layers, for example, eight layers (See Journal of applied Physics, Vol. 92, No. 12, pp. 7462-7468, 15 Dec. 2002, for example).
FIG. 18 is a conceptual sectional view showing an example of a conventional QDIP. In this example, an n-type GaAs bottom electrode layer 52 is formed on a semi-insulating GaAs substrate 51, and quantum dot layers 53 are stacked approximately in 6 to 20 cycles (layers) on the n-type GaAs bottom electrode layer 52. Each of the quantum dot layers 53 is constituted by embedding plural InAs quantum dots 55 in an i-type GaAs layer 54. Further, an n-type GaAs top electrode layer 56 is formed on the uppermost quantum dot layer 53. Furthermore, electrodes 57 and 58 made of AuGe/Ni/Au are respectively formed on the n-type GaAs bottom electrode layer 52 and the n-type GaAs top electrode layer 56. Such a stacked structure can be obtained if the n-type GaAs bottom electrode layer 52, the quantum dot layers 53 approximately in 6 to 20 cycles, and the n-type GaAs top electrode layer 56 are formed in sequence on the semi-insulating GaAs substrate 51 and thereafter partially etching the n-type GaAs top electrode layer 56 and the quantum dots layers 53 so as to expose the n-type GaAs bottom electrode layer 52.
Next, the operation of the conventional QDIP shown in FIG. 18 will be described.
FIG. 19 is a diagram showing a conduction-band edge profile when no bias is applied to the conventional QDIP shown in FIG. 18. When no bias is applied, no impurity is added to the stack of the quantum dot layers 53, so that electrons are supplied from the n-type GaAs bottom electrode layer 52 and the n-type GaAs top electrode layer 56. However, quantum levels (a ground level 61 and an excited level 62) formed in the InAs quantum dot 55 is located at energy levels lower than a conduction-band edge of GaAS, so that the supplied electrons are relaxed to the quantum levels 61 and 62.
Since Fermi energy Ef is defined to be equal to energy at which the proportion of occupation of electrons in some state reduces to half, in this case, the ground level 61 corresponds to the energy.
When a system is kept in thermal equilibrium, the Fermi energy Ef is fixed in the entire system. Hence, conduction-band edges of the InAs quantum dots 55 rise such that conduction-band edges of the electrode layers 52 and 56, which are at a Fermi energy level, and the ground levels 61 of the InAs quantum dots 55 substantially match each other.
However, in the quantum dot layers 53 located near the n-type GaAs bottom electrode layer 52 and the n-type GaAs top electrode layer 56, excessive electrons are supplied from the electrode layers 52 and 56, so that the Fermi energy reaches the excited level 62. As a result, the rise of the conduction-band is small.
FIG. 20 is a diagram showing a conduction-band edge profile when a bias is applied to the conventional QDIP shown in FIG. 18. Here, it is assumed that an electric field is applied uniformly to the stack of the quantum dot layers 53. When a bias is applied, a difference in potential applied by a power source 59 exists between a pair of electrodes 57 and 58, and hence if infrared light is irradiated from the outside to excite electrons, these electrons flow between the n-type GaAs bottom electrode layer 52 and the n-type GaAs top electrode layer 56. Namely, a photocurrent is generated.
On the other hand, even in a state where the infrared light is not irradiated, due to a difference in potential applied between the n-type GaAs bottom electrode layer 52 and the n-type GaAs top electrode layer 56, an electron 63 located in the low-potential side electrode layer (n-type GaAs bottom electrode layer 52) flows between the electrode layers 52 and 56. Namely, the electron 63 which has flowed from the low-potential side electrode layer (n-type GaAs bottom electrode layer 52) regards a conduction-band edge of the i-type GaAs layer 54 which is several layers away therefrom as the highest barrier, and when the electron 63 goes over the conduction-band edge, a dark current flows. This dark current flows also when the infrared light is irradiated.
Since the performance of the photo detector is mostly dependent on the ratio of a photocurrent to a dark current, it is a requirement to reduce the dark current in order to improve the performance of the detector.
From the above-described mechanism, the higher the barrier, the smaller the dark current becomes, and therefore, heightening the barrier is a means of reducing the dark current. Hence, it is thought that if the barrier is heightened by embedding the InAs quantum dots 55 in a semiconductor layer such as an AlGaAs layer, which has a larger band gap than the i-type GaAs layer 54, in place of the i-type GaAs layer 54, the reduction in dark current becomes possible.
However, although in the growth of quantum dots, a growth temperature of about 500° C. is required, the AlGaAs layer is generally grown at about 600° C. Therefore, in terms of the optimization of the growth condition and so on, it is difficult to use the AlGaAs layer as the layer in which the quantum dots are embedded.